Production strings are typically connected to a seal bore packer extension in a way that energizes a metal to metal seal at the lower end of a seal bore with opposed chevron seal stacks above the metal to metal seals that seal between the anchor sub and the balance of the seal bore. The connection is held together by a two-step threaded connection. One issue with this arrangement is that the two step thread has sufficient play axially such that if the production string is put into tension the metal seal may lose contact leaving only the chevron seal stack as the sole seal. This condition can create issues with regulations that require dual seals.
FIG. 1 illustrates this arrangement. Packer extension 10 has a seal bore 12. An anchor sub 14 has a metal to metal seal 16 and opposed chevron seal stacks 18. As a result of making up two-step thread 20 the metal seal 16 is energized and the seal stack 18 is against the seal bore 12. The issue with this design is that there is axial play in the two step thread 20 which combined with tension on the anchor sub 14 from the production string that supports it and is not shown has the result of undermining the metal to metal seal. This leaves the seal stack 18 as the sole operating seal and regulations or operator policies require dual seals for arrangements shown in FIG. 1.
The present invention represents a new take on dealing with this issue. The seal bore in the packer extension is made optional and the anchor sub is eliminated in favor of an external thread on the lower end of the production string that engages directly to the packer extension. The thread is preferably a VAM type premium thread, or similar premium or tapered thread meeting the performance criteria all being collectively referred to as premium thread, so that a metal to metal seal results and the seal stack is eliminated. The VAM thread is combined with a left handed square thread to engage a retrieval tool after the packer is released to relax for retrieval such as by breaking a shear member with a latching collet on a retrieval tool or simply cutting through the packer mandrel to release it. These and other aspects of the present invention will be more readily apparent from a review of the description of the preferred embodiment and the associated drawings while recognizing that the full scope of the invention is to be determined by the appended claims.